Bundles of high strength fibers of non-metals and semi-metals, such as carbon, boron, silicon carbide, and the like, in the form of filaments, mats, cloths and chopped strands are known to be useful in reinforcing metals and organic polymeric materials. Articles comprising metals or plastics reinforced with such fibers find wide-spread use in replacing heavier components made of lower strength conventional materials such as aluminum, steel, titanium, vinyl polymers, nylons, polyesters, etc., in aircraft, automobiles, office equipment, sporting goods, and in many other fields.
A common problem in the use of such fibers, and also glass, asbestos, and others, is a seeming lack of ability to translate the properties of the high strength fibers to the material to which ultimate and intimate contact is to be made.
The problem is manifested in a variety of ways: for example, if a length of high strength carbon fiber yarn is enclosed lengthwise in the center of a rod formed from solidified molten lead, and the rod is pulled until broken, the breaking strength will be less than expected from the rule of mixtures, and greater than that of a rod formed from lead alone, due to the mechanical entrapment of the fibers.The lack of reinforcement is entirely due to poor translation of strength between the carbon fibers and the lead. The same thing happens if an incompatible high strength fiber is mixed with a plastic material. If some types of carbon fibers, boron fibers, silicon carbide fibers, and the like in the forms of strands, chopped strands, non-woven mats, felts, papers, etc. or woven fabrics are mixed with organic polymeric substances, such as phenolics, styrenics, epoxy resins, polycarbonates, and the like, or mixed into molten metals, such as lead, aluminum, titanium, etc., they merely fill them without providing any reinforcement, and in many cases even cause physical properties to deteriorate.
All of these problems are generally recognized now, after years of research, to result from the need to insure adequate bonding between the high strength fiber and the so-called matrix material, the metal or plastic sought to be reinforced. It is also known that bonding can be improved with careful attention to the surface layer on each macro-micro filament or fibril in the material selected for use. Glass filaments, for example, are flame cleaned and then sized with a plastic-compatible organosilane to produce reinforcements uniquely suitable for plastics.
Such techniques do not work well with other fibrous materials and, for obvious reasons, are not suitable for carbon fibers, which would not surface texture, and which have different boundary layers.
High strength carbon fibers are made by heating polymeric fiber, e.g., acrylonitrile polymers or copolymers, in two stages, one to remove volatiles and carbonize and another to convert amorphous carbon into crystalline carbon. During such procedure, it is known that the carbon changes from amorphous to single crystal then orients into fibrils. If the fibers are stretched during the graphitization, then high strength fibers are formed. This is critical to the formation of the boundary layer, because as the crystals grow, there are formed high surface energies, as exemplified by incomplete bonds, edge-to-edge stresses, differences in morphology, and the like. It is also known that the new carbon fibrils in this form can scavenge nascent oxygen from the air, and even organic materials, to produce non-carbon surface layers which are firmly and chemically bonded thereto, although some can be removed by solvent treating, and there are some gaps or open spaces in the boundary layers. Not unlike the contaminants or uncleaned, unsized glass filaments, these boundary layers on carbon fibers are mainly responsible for failure to achieve reinforcement with plastics and metals.
Numerous unsuccessful attempts have been reported to provide such filaments, especially carbon filaments, in a form uniquely suitable for reinforcing metals and plastics. Most have involved depositing layers of metals, especially nickel and copper as thin surface layers on the filaments. Such a composite fiber was then to be used in a plastic or metal matrix. The metals in the prior art procedures have been vacuum deposited, electrolessly deposited, and electrolytically deposited, but the resulting composites were not suitable.
Vacuum deposition, e.g., of nickel, U.S. Pat. No. 4,132,828, made what appears to be a continuous coating, but really isn't because the vacuum deposited metal first touches the fibrils through spaces in the boundary layer, then grows outwardly like a mushroom, then joins away from the surface, as observed under a scanning electron microscope as nodular nucleation. If the fiber is twisted, such a coating will fall off. The low density non-crystalline deposit limits use.
Electroless nickel baths have also been employed to plate such fibers but again there is the same problem, the initial nickel or other electroless metal seeds only small spots through holes in the boundary layer, then new metal grows up like a mushroom and joins into what looks like a continuous coating, but it too will fall off when the fiber is twisted. The intermetallic compound is very locally nucleated and this, too, limits use. In the case of both vacuum deposition and electroless deposition, the strength of the metal-to-core bond is always substantially less than one-tenth that of the tensile strength of the metal deposit itself.
Finally, electroplating with nickel and other metals is also featured in reported attempts to provide carbon fibers with a metal layer to make compatible with metals and plastics, e.g., R. V. Sara, U.S. Pat. No. 3,622,283. Short lengths of carbon fibers were clamped in a battery clip, immersed in an electrolyte, and electroplated with nickel. When the plated fibers were put into a tin metal matrix, the fibers did not translate their strength to the matrix to the extent expected from the rule of mixtures. When fibers produced by such a process are sharply bent, on the compression side of the bend there appear a number of transverse cracks and on the tension side of the bend the metal breaks and flakes off. If the metal coating is mechanically stripped, and the reverse side is examined under a high-power microscope, there is either no replica or at best only an incomplete replica of the fibril, the replica defined to the 40 angstrom resolution of the scanning electron microscope. The latter two observations are strongly suggestive that failure to reinforce the tin matrix was due to poor bonding between the carbon and the nickel plating. In these cases, the metal to core bond strength is no greater than one-half of the tensile strength on at most 10% of the fibers, and substantially less than one-tenth on the remaining 90%.
It has now been discovered that if electroplating is selected, and if a very high order of external voltage is applied, much higher than was thought to be achievable in the prior art, uniform, continuous adherent, thin metal coatings can be provided to reinforcing fibrils, especially carbon fibrils. The voltage must be high enough to provide energy sufficient to push the metal ions through the boundary layer to provide uniform nucleation with the fibrils directly. Composites of yarns or tows comprising the thin metal coatings on fibers, woven cloth, yarns, and the like, according to this invention can be knotted and folded without the metal flaking off. The composites are distinguishable from any of the prior art because they can be sharply bent without the fibrils slipping through a tube of the metal, as observed with electroless metal or vacuum deposited composites and sharply bending them, especially with nickel, produces neither transverse cracking ("alligatoring") on the compression side of the bend nor breaking and flaking when the elastic limit of the metal is exceeded on the tension side of the bend. In other words, the composites of the present invention are distinguishable from those of the prior art because (i) they are continuous, (ii) the majority of the composite fibers are uniformly metal coated; and (iii) the bond strength (metal-to-core) on the majority of fibers is at least about 10 percent of the tensile strength of the metal deposit, preferably not substantially less than about 25 percent, especially preferably not substantially less than about 50 percent. In the most preferred embodiments, the metal-to-core bond strength will be not substantially less than about 90 percent of the tensile strength of the metal deposit. Highest properties will be achieved with yarns or tows of composite fibers in which the metal-to-core bond strength approaches about 99 percent of the tensile strength of the metal, and special mention is made of these.
Articles made by adding the yarns or tows of the present invention to a matrix forming material also distinguish from the prior art because they are strongly reinforced. In addition, the articles possess other advantages, for example, they dissipate electrical charges and if certain innocuous metals are used in the coatings, e.g., gold and platinum, they will not be rejected when implanted into the body.